Positive electrode active material and lithium secondary battery including the same

ABSTRACT

Disclosed herein are a positive electrode active material including at least one selected from among compounds represented by Formula 1 below and a lithium secondary battery including the same that is capable of improving lifetime characteristics and rate characteristics while exhibiting excellent safety: xLi 2 M y Mn (1-y) O 3-z A z *(1−x)LiM′O 2-z′ A′ z′  (1), where M is at least one element selected from a group consisting of Ru, Mo, Nb, Te, Re, Ir, Pt, Cr, S, W, Os, and Po, M′ is at least one element selected from a group consisting of Ni, Ti, Co, Al, Mn, Fe, Mg, B, Cr, Zr, Zn, and second row transition metals, A and A′ are each independently a negative monovalent or divalent anion, and 0&lt;x&lt;1, 0.3&lt;y&lt;1, 0≦z&lt;0.5, and 0≦z′&lt;0.5.

TECHNICAL FIELD

The present invention relates to a positive electrode active materialand a lithium secondary battery including the same that is capable ofimproving lifetime characteristics and rate characteristics whileexhibiting excellent safety.

BACKGROUND ART

In recent years, with increased concerns about environmental problems,much research has been carried out into electric vehicles (EV) andhybrid electric vehicles (HEV), which are capable of substituting forvehicles using fossil fuel, such as gasoline and diesel oil, which causeair pollution. Nickel-metal hydride (Ni—MH) secondary batteries havebeen mainly used as a power source for the electric vehicles and thehybrid electric vehicles. On the other hand, much research has also beencarried out into lithium secondary batteries having high energy density,high discharge voltage, and high output stability, and some of thelithium secondary batteries are now commercialized.

In particular, it is necessary for lithium secondary batteries used forelectric vehicles to exhibit high energy density and to provide highoutput within a short time. In addition, it is also necessary forlithium secondary batteries used for electric vehicles to be used for 10years or more under severe conditions. For these reasons, lithiumsecondary batteries used for electric vehicles require higher safety andlonger lifespan than conventional small-sized lithium secondarybatteries. In addition, secondary batteries used for electric vehicles(EV) and hybrid electric vehicles (HEV) require excellent ratecharacteristics and excellent power characteristics based on theoperation conditions of a vehicle.

Carbon materials are mainly used for negative electrode active materialsof a lithium secondary battery, and the use of lithium metals and sulfurcompounds is under consideration. In addition, lithium-containing cobaltoxides, such as LiCoO₂, are mainly used for positive electrode activematerials of a lithium secondary battery, and the use oflithium-containing manganese oxides, such as LiMnO₂, having a layeredcrystal structure, LiMn₂O₄, having a spinel crystal structure, andlithium-containing nickel oxides, such as LiNiO₂, is also underconsideration.

Among the positive electrode active materials, LiCoO₂ is widely used dueto its excellent lifetime characteristics and high charge and dischargeefficiency, but has low structural stability. In addition, due to theresource limitations of cobalt, which is a raw material, LiCoO₂ isexpensive, and therefore price competitiveness is low, whereby themassive use thereof as power sources in fields such as electric vehiclesis limited.

LiNiO₂ is relatively inexpensive, and makes it possible for a battery tohave high discharge capacity. However, phase transition abruptly occursin the crystal structure of LiNiO₂ depending on the change in volume ofthe battery caused by charge and discharge of the battery. In addition,when LiNiO₂ is exposed to air and moisture, the safety of LiNiO₂ isabruptly lowered.

In addition, lithium manganese oxides, such as LiMnO₂ and LiMn₂O₄,exhibit excellent thermal safety and are inexpensive. However, the useof lithium manganese oxides entails a small charge capacity, poor cyclecharacteristics, and poor high-temperature characteristics.

For these reasons, much research has been carried out into new positiveelectrode active materials that have structures other than theabove-mentioned structures.

For example, research has been carried out into an oxide containingexcessive lithium, e.g. a lithium transition metal oxide containing ahigh content of Mn, wherein the content of lithium is higher than thecontent of the transition metal such that a high capacity of 270 or moremAh/g is exhibited at a high voltage of 4.5 V or higher.

However, the oxide containing excessive lithium has a high irreversiblecapacity. Furthermore, in addition to lithium, oxygen escapes from theactive material structure at the time of high-voltage activation toutilize excess lithium. As a result, the active material structure maycollapse, and a voltage sagging phenomenon may occur, whereby thedeterioration of the battery cell may be accelerated.

Meanwhile, many researchers have proposed methods of using a positiveelectrode active material containing Li₂MnO₃ in a layered structure inorder to secure the structural stability of the positive electrodeactive material having such a layered structure. In this case, thepositive electrode active material contains a large amount of Mn. As aresult, the positive electrode active material is very cheap andexhibits large capacity and high stability at high voltage. After theactivation of a flat range of 4.4 V to 4.6 V, however, the transitionfrom the layered structure to the spinel structure is performed, andtherefore the contact between domains is weakened. As a result, thepositive electrode active material may be excessively structurallychanged, whereby the improvement of electrical properties may not besatisfied.

That is, the structure of a secondary battery that exhibits desiredlifetime characteristics and safety has not yet been proposed.

DISCLOSURE Technical Problem

The present invention has been made to solve the above problems andother technical problems that have yet to be resolved.

As a result of a variety of extensive and intensive studies andexperiments to solve the problems described above, the inventors of thepresent application have developed a positive electrode active materialfor secondary batteries including compounds represented by Formula 1, aswill be described hereinafter, and have found that the positiveelectrode active material exhibits structural stability during thecharge and discharge of a secondary battery, and therefore a secondarybattery including the positive electrode active material exhibitsexcellent lifetime characteristics and rate characteristics as well assafety. The present invention has been completed based on thesefindings.

Technical Solution

In accordance with one aspect of the present invention, the above andother objects can be accomplished by the provision of a positiveelectrode active material including at least one selected from amongcompounds represented by Formula 1 below:xLi₂M_(y)Mn_((1-y))O_(3-z)A_(z)*(1−x)LiM′O_(2-z′)A′_(z′) (1), where M isat least one element selected from the group consisting of Ru, Mo, Nb,Te, Re, Ir, Pt, Cr, S, W, Os, and Po, M′ is at least one elementselected from the group consisting of Ni, Ti, Co, Al, Mn, Fe, Mg, B, Cr,Zr, Zn, and second row transition metals, A and A′ are eachindependently a negative monovalent or divalent anion, and 0<x<1,0.3<y<1, 0≦z<0.5, and 0≦z′<0.5.

In the case in which a positive electrode active material containingexcessive lithium and a high content of manganese is used, as previouslydescribed, the positive electrode active material provides a largecapacity compared to the price thereof. However, oxygen escapes from theactive material structure in addition to lithium at the time ofhigh-voltage activation to utilize excess lithium. As a result, theactive material structure may collapse, whereby the electrochemicalperformance of the positive electrode active material may bedeteriorated.

The inventors of the present application have conducted much researchinto the manufacture of a positive electrode active material that has astable crystal structure at high voltage while exhibiting excellentcapacity and rate characteristics, and have found that in the case inwhich a specific element is substituted for Mn, the above problem can besolved.

Specifically, in the case in which a specific element defined as M inFormula 1 above is substituted for Mn, oxygen is prevented from escapingthe active material structure at high voltage, thereby securingstructural stability and thus exhibiting excellent capacity and ratecharacteristics. In addition, voltage necessary for high-voltageactivation to utilize excess lithium may be lowered, with the resultthat the generation of gas is prevented due to the restraint of thedecomposition of an electrolytic solution and the generation of oxygenradicals at high voltage, whereby the safety of the battery is improved.

In the present invention, M may be an element having a greater ionradius than lithium. Specifically, M may be at least one elementselected from the group consisting of Ru, Mo, S, W, Os, and Po. Morespecifically, M may be Ru.

In Formula 1 above, M′ may be a transition metal that satisfies theabove conditions. For example, M′ may be Mn_(a)M″_(1-a) (where M″ is atleast one element selected from a group consisting of Ni, Ti, Co, Al,Fe, Mg, B, Cr, Zr, Zn, and second row transition metals, and 0<a<1). Inparticular, M″ may be a combination of at least two elements selectedfrom the group consisting of Ni, Ti, Co, Al, Fe, Mg, B, Cr, Zr, and Zn.

Specifically, M′ may be Mn_(a)Ni_(b)Co_(c) (where 0<a<1, 0<b<1, 0<c<1,and a+b+c=1). In the case in which M′ includes a combination ofmanganese, nickel, and cobalt, the defects of respective elements may becompensated for, whereby excellent effects may be achieved.

In addition, a, b, and c in M′ may have various values within a rangethat satisfies the above conditions. Specifically, a may have valueswithin a range of 0<a<0.5. More specifically, a may have values within arange of 0.1<a<0.4.

In Formula 1 above, x indicates a molar ratio. If x is too large,electron conductivity is lowered even though charge and dischargecapacity is increased. Specifically, x may have values within a range of0<x≦0.7. More specifically, x may have values within a range of0.2<x≦0.6. Most specifically, x may be 0.5.

In Formula 1 above, if the content of y is 0.3 or less, desired effectsmay not be obtained. If the content of y is 1, the capacity may bereduced. Within the range in which the layered crystal structure ismaintained, therefore, y may have the following range of values.Specifically, y may have values within a range of 0.4<y<1. Morespecifically, y may have values within a range of 0.6≦y<1. Mostspecifically, y may be 0.6.

In Formula 1 above, a predetermined amount of other anions may besubstituted for oxygen ions within the range in which A and A′independently satisfy the above conditions. Specifically, A and A′ mayeach be at least one element selected from the group consisting ofhalogen elements, such as F, Cl, Br, and I, sulfur, and nitrogen.

The coupling force with the transition metal is increased and thestructural transition of the active material is prevented by thesubstitution of anions, with the result that the lifespan of the batterymay be increased. If 0.5 or more moles of anions are substituted,however, the structure of the compound is not stably maintained, withthe result that lifetime characteristics are lowered, which is notdesirable. Consequently, the substituted amounts z and z′ of the anionsmay have values within ranges of 0≦z≦0.2 and 0≦z′≦0.2. Morespecifically, the substituted amounts z and z′ of the anions may havevalues within ranges of 0≦z≦0.1 and 0≦z′≦0.1.

The compounds represented by Formula 1 above may be layered compositesor solid solutions. According to circumstances, the layered compositesand the solid solutions may be mixed with each other.

M may be uniformly distributed on the surface of the compound and withinthe compound. According to circumstances, M may have a gradient ofconcentration from the surface of the compound into the interior of thecompound. The content of M may be increased toward the interior of thecompound. On the assumption that a portion equivalent to 1% of the rangeof the compound from the outermost surface to the middle of the compoundbased on the particle diameter of the compound is defined as the surfaceof the compound, 0.1 to 20 weight %, specifically 0.2 to 10 weight %, ofthe total weight of M may be distributed on the surface of M.

The method of manufacturing the positive electrode active material isnot particularly restricted as long as the molar ratio of lithium andmanganese represented by

Formula 1 is maintained. For example, the positive electrode activematerial may be manufactured through a reaction between a transitionmetal hydroxide precursor and lithium carbonate. Specifically, atransition metal hydroxide precursor having a composition sufficient toobtain an oxide containing an excessive amount of lithium after reactionis prepared, the prepared transition metal hydroxide precursor is mixedwith a lithium-containing precursor and M such that they react with oneanother, and the mixture may be plasticized at a temperature of about800 to 1200° C. for about 8 to 24 hours to manufacture a positiveelectrode active material.

In accordance with another aspect of the present invention, there isprovided a positive electrode formed by applying the positive electrodeactive material with the above-stated construction to a currentcollector.

The positive electrode may further include general lithium transitionmetal oxides that do not satisfy the above conditions as a positiveelectrode active material, in addition to the compounds represented byFormula 1 above. Such general lithium transition metal oxides mayinclude oxides including only one selected from among Ni, Co, and Mn andtwo or more selected from among Ni, Co, and Mn. For example, all lithiumtransition metal oxides well-known in the art to which the presentinvention pertains may be included. In this case, the compoundsrepresented by Formula 1 above may be at least 30 weight %, specifically50 weight %, of the total weight of the positive electrode activematerial.

The positive electrode according to the present invention may bemanufactured by mixing a positive electrode mixture including thepositive electrode active material, a conductive agent, and a binderwith a predetermined solvent, such as water or NMP, to manufacture aslurry, applying the slurry to a positive electrode current collector,and drying and pressing the slurry.

At least one selected from the group consisting a viscosity controllingagent and a filler may be further included in the positive electrodemixture as needed.

Generally, the positive electrode current collector has a thickness of 3to 500 μm. The positive electrode current collector is not particularlyrestricted so long as the positive electrode current collector exhibitshigh conductivity while the positive electrode current collector doesnot induce any chemical change in a battery to which the positiveelectrode current collector is applied. For example, the cathode currentcollector may be made of stainless steel, aluminum, nickel, titanium, orplastic carbon. Alternatively, the positive electrode current collectormay be made of aluminum or stainless steel, the surface of which istreated with carbon, nickel, titanium, or silver. The positive electrodecurrent collector may have a micro uneven pattern formed at the surfacethereof so as to increase the adhesive strength of the positiveelectrode active material. The positive electrode current collector maybe configured in various forms, such as a film, a sheet, a foil, a net,a porous body, a foam body, and a non-woven fabric body.

The conductive agent, which is a component for further improving theconductivity of the positive electrode active material, may be added sothat the conductive agent has 0.01 to 30 weight % based on the totalweight of the positive electrode mixture. The conductive agent is notparticularly restricted so long as the conductive agent exhibits highconductivity while the conductive agent does not induce any chemicalchange in a battery to which the conductive agent is applied. Forexample, graphite, such as natural graphite or artificial graphite;carbon black, such as carbon black, acetylene black, Ketjen black,channel black, furnace black, lamp black, or summer black; conductivefiber, such as carbon fiber or metallic fiber; metallic powder, such ascarbon fluoride powder, aluminum powder, or nickel powder; conductivewhisker, such as zinc oxide or potassium titanate; conductive metaloxide, such as titanium oxide; or polyphenylene derivatives may be usedas the conductive agent.

The binder is a component that assists in binding between the positiveelectrode active material and the conductive agent and in binding withthe positive electrode current collector. The binder is generally addedin an amount of 1 to 30 weight % based on the total weight of thecompound including the positive electrode active material. As examplesof the binder, there may be used polyvinylidene fluoride, polyvinylalcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose,regenerated cellulose, polyvinyl pyrollidone, tetrafluoroethylene,polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM),sulfonated EPDM, styrene butadiene rubber, fluoro rubber, and variouscopolymers.

The viscosity controlling agent is a component for controlling theviscosity of the electrode mixture so as to facilitate mixing of theelectrode mixture and coating thereof on the current collector. Theviscosity controlling agent may be added in an amount of up to 30 weight% based on the total weight of the electrode mixture. Examples of theviscosity controlling agent include carboxymethylcellulose andpolyvinylidene fluoride. However, the present invention is not limitedthereto. According to circumstances, the above-described solvent mayalso function as the viscosity controlling agent.

The filler is an optional component used to inhibit the expansion of thepositive electrode. There is no particular limit to the filler so longas the filler is made of a fibrous material while the filler does notcause chemical changes in a battery to which the filler is applied. Asexamples of the filler, there may be used olefin polymers, such aspolyethylene and polypropylene; and fibrous materials, such as glassfiber and carbon fiber.

In accordance with another aspect of the present invention, there isprovided a lithium secondary battery including the positive electrode, anegative electrode, a separator, and a non-aqueous electrolytic solutioncontaining lithium salt.

The negative electrode is manufactured by applying a negative electrodemixture including a negative electrode active material to a negativeelectrode current collector and drying the negative electrode mixture.The conductive agent, the binder, etc., which were previously described,may be added to the negative electrode mixture as needed.

In general, the negative electrode current collector has a thickness of3 to 500 μm. The negative electrode current collector is notparticularly restricted so long as the negative electrode currentcollector exhibits high conductivity while the negative electrodecurrent collector does not induce any chemical change in a battery towhich the negative electrode current collector is applied. For example,the negative electrode current collector may be made of copper,stainless steel, aluminum, nickel, titanium, or plastic carbon.Alternatively, the negative electrode current collector may be made ofcopper or stainless steel, the surface of which is treated with carbon,nickel, titanium, or silver, or an aluminum-cadmium alloy. In the samemanner as in the positive electrode current collector, the negativeelectrode current collector may have a micro uneven pattern formed atthe surface thereof so as to increase the adhesive force of the negativeelectrode active material. The negative electrode current collector maybe configured in various forms, such as a film, a sheet, a foil, a net,a porous body, a foam body, and a non-woven fabric body.

The negative electrode active material may be, for example, carbon andgraphite materials such as natural graphite, artificial graphite,expandable graphite, carbon fiber, hard carbon, carbon black, carbonnanotubes, fullerenes, and activated carbon; metals alloyable withlithium such as Al, Si, Sn, Ag, Bi, Mg, Zn, In, Ge, Pb, Pd, Pt, and Tiand compounds including these elements; complexes of metals andcompounds thereof and complexes of carbon and graphite materials; andlithium-containing nitrides. Thereamong, a carbon-based active material,a tin-based active material, a silicon-based active material, or asilicon-carbon-based active material is more preferable and may be usedalone or in combination of two or more thereof.

The separator is interposed between the positive electrode and thenegative electrode. As the separator, for example, an insulative thinfilm exhibiting high ion permeability and high mechanical strength maybe used. The separator generally has a pore diameter of 0.01 to 10 μmand a thickness of 5 to 300 μm. As the material for the separator, forexample, a sheet or non-woven fabric made of olefin polymer, such aspolypropylene, which exhibits chemical resistance and hydrophobicity,glass fiber, or polyethylene is used. In the case in which a solidelectrolyte, such as polymer, is used as an electrolyte, the solidelectrolyte may also function as the separator.

The non-aqueous electrolytic solution containing lithium salt iscomposed of an electrolytic solution and lithium salt. As theelectrolytic solution, a non-aqueous organic solution, an organic solidelectrolyte, or an inorganic solid electrolyte may be used.

As examples of the non-aqueous organic solution, mention may be made ofnon-protic organic solvents, such as N-methyl-2-pyrollidinone, propylenecarbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate,diethyl carbonate, gamma-butyro lactone, 1,2-dimethoxy ethane,tetrahydroxyfuran, 2-methyl tetrahydrofuran, dimethylsulfoxide,1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile,nitromethane, methyl formate, methyl acetate, phosphoric acid triester,trimethoxy methane, dioxolane derivatives, sulfolane, methyl sulfolane,1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives,tetrahydrofuran derivatives, ether, methyl propionate, and ethylpropionate.

As examples of the organic solid electrolyte, mention may be made ofpolyethylene derivatives, polyethylene oxide derivatives, polypropyleneoxide derivatives, phosphoric acid ester polymers, poly agitationlysine, polyester sulfide, polyvinyl alcohols, polyvinylidene fluoride,and polymers containing ionic dissociation groups.

As examples of the inorganic solid electrolyte, mention may be made ofnitrides, halides, and sulphates of lithium (Li), such as Li₃N, LiI,Li₅NI₂, Li₃N—LiI—LiOH, LiSiO₄, LiSiO₄—LiI—LiOH, Li₂SiS₃, Li₄SiO₄,Li₄SiO₄—LiI—LiOH, and Li₃PO₄—Li₂S—SiS₂.

The lithium salt is a material that is readily soluble in theabove-mentioned non-aqueous electrolytic solution, and may include, forexample, LiCl, LiBr, LiI, LiClO₄, LiBF₄, LiB₁₀Cl₁₀, LiPF₆, LiCF₃SO₃,LiCF₃CO₂, LiAsF₆, LiSbF₆, LiAlCl₄, CH₃SO₃Li, CF₃SO₃Li, (CF₃SO₂)₂NLi,chloroborane lithium, lower aliphatic carboxylic acid lithium, lithiumtetraphenyl borate, and imide.

In addition, in order to improve charge and discharge characteristicsand flame retardancy, for example, pyridine, triethylphosphite,triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphorictriamide, nitrobenzene derivatives, sulfur, quinone imine dyes,N-substituted oxazolidinone, N,N-substituted imidazolidine, ethyleneglycol dialkyl ether, ammonium salts, pyrrole, 2-methoxy ethanol,aluminum trichloride, or the like may be added to the non-aqueouselectrolytic solution. According to circumstances, in order to impartincombustibility, the non-aqueous electrolytic solution may furtherinclude halogen-containing solvents, such as carbon tetrachloride andethylene trifluoride. Furthermore, in order to improve high-temperatureretention characteristics, the non-aqueous electrolytic solution mayfurther include carbon dioxide gas. In addition, fluoro-ethylenecarbonate (FEC), propene sultone (PRS), fluoro-propylene carbonate(FPC), etc. may be further included.

The secondary battery according to the present invention may be used asa unit battery of a battery module that is a power source of a middle orlarge-sized device, which particularly requires stability against hightemperatures, long cycle and high rate characteristics, etc.

Specifically, the middle or large-sized device may be a power tooldriven by a battery-based motor, an electric automobile, such as such asan electric vehicle (EV), a hybrid electric vehicle (HEV), or a plug-inhybrid electric vehicle (PHEV), an electric two-wheeled vehicle, such asan electric bicycle (E-bike) or an electric scooter (E-scooter), or anelectric golf cart. However, the present invention is not limitedthereto.

In accordance with a further aspect of the present invention, there isprovided a middle or large-sized battery pack using the secondarybattery as a unit battery. The general structure and manufacturingmethod of the middle or large-sized battery pack are well known in theart to which the present invention pertains, and therefore a detaileddescription thereof will be omitted.

BRIEF DESCRIPTION OF DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a graph showing the results of charge and discharge in secondcycles of batteries according to Experimental Example 1; and

FIG. 2 is capacity differentiation curves in second to seventh cycles ofbatteries according to Experimental Example 2.

BEST MODE

Now, the present invention will be described in more detail withreference to the following examples. These examples are provided onlyfor illustrating the present invention and should not be construed aslimiting the scope and spirit of the present invention.

EXAMPLE 1

Transition metal precursors, NiCO₃, MnCO₃, RuO₂, and Li₂CO₃, were placedin a planetary mill in the state in which the molar ratio of thetransition metal precursors was 1:1.8:1.2:3 based on a stoichiometricratio, and were mixed at a speed of 400 rpm for about 5 hours to obtaina mixture. The mixture was plasticized at an air atmosphere of 900° C.for 8 hours to manufacture a positive electrode active material dopedwith Ru, 0.5Li₂Ru_(0.6)Mn_(0.4)O₃*0.5LiNi_(0.5)Mn_(0.5)O₂.

Comparative Example 1

0.5Li₂Ru_(0.3)Mn_(0.7)O₃*0.5LiNi_(0.5)Mn_(0.5)O₂ was manufactured in thesame manner as in Example 1 except that transition metal precursors,NiCO₃, MnCO₃, RuO₂, and Li₂CO₃, were mixed in the state in which themolar ratio of the transition metal precursors was 1:2.4:0.6:3 based ona stoichiometric ratio.

Comparative Example 2

0.5Li₂MnO₃*0.5LiNi_(0.5)Mn_(0.5)O₂ was manufactured in the same manneras in Example 1 except that Ru salt was not used.

Experimental Example 1

7 weight % of denka black, as a conductive agent, and 6 weight % ofPVDF, as a binder, were added to 87 weight % of the positive electrodeactive materials manufactured according to Example 1 and ComparativeExamples 1 and 2 to manufacture positive electrode active mixtures. NMPwas added to the manufactured positive electrode active mixtures tomanufacture slurries. The manufactured slurries were applied to positiveelectrode current collectors, and were then pressed and dried tomanufacture positive electrodes for secondary batteries. Porouspolyethylene separators were interposed between the positive electrodesand negative electrodes based on lithium metal, and then lithiumelectrolytic solutions were injected to manufacture coin type lithiumhalf-cell batteries.

The results of charge and discharge in second cycles of the coin typelithium half-cell batteries are shown in FIG. 1.

Referring to this figure, it can be seen that the battery according toExample 1 exhibits high capacity at a lower voltage than the batteriesaccording to Comparative Examples 1 and 2 and that a voltage saggingphenomenon is reduced.

Experimental Example 2

The lithium half-cell batteries manufactured according to ExperimentalExample 1 using the positive electrode active materials manufacturedaccording to Example 1 and Comparative Examples 1 and 2 were charged anddischarged up to seventh cycles. Capacity differentiation curves insecond to seventh electrochemical cycles of the batteries are shown inFIG. 2.

Referring to this figure, it can be seen that the battery according toExample 1 has a lower activation level in a colored portion in adotted-line box than the batteries according to Comparative Examples 1and 2, whereby a voltage sagging phenomenon is reduced even when aplurality of charge and discharge cycles is performed.

INDUSTRIAL APPLICABILITY

As is apparent from the above description, a positive electrode activematerial according to the present invention contains excessive lithiumand a predetermined content of specific elements. Even at the time ofhigh-voltage activation to utilize excess lithium, therefore, escape ofoxygen may be prevented, whereby it is possible to secure structuralstability. Consequently, it is possible to restrain voltage sagging dueto the structural change of the positive electrode active materialduring cycles, thereby improving lifetime characteristics.

In addition, the irreversible capacity of the positive electrode activematerial may be reduced through sufficient utilization of excesslithium, thereby improving rate characteristics while exhibitingexcellent charge and discharge efficiency.

Furthermore, voltage necessary for high-voltage activation may belowered, with the result that it is possible to prevent the generationof oxygen radicals due to the restraint of the decomposition of anelectrolytic solution, whereby the safety of a battery may be improved.

Although the exemplary embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A positive electrode active material comprising at least one selectedfrom among compounds represented by Formula 1 below:xLi₂M_(y)Mn_((1-y))O_(3-z)A_(z)*(1−x)LiM′O_(2-z′)A′_(z′)  (1), where Mis at least one element selected from a group consisting of Ru, Mo, Nb,Te, Re, Ir, Pt, Cr, S, W, Os, and Po, M′ is at least one elementselected from a group consisting of Ni, Ti, Co, Al, Mn, Fe, Mg, B, Cr,Zr, Zn, and second row transition metals, A and A′ are eachindependently a negative monovalent or divalent anion, and 0<x<1,0.3<y<1, 0≦z<0.5, and 0≦z′<0.5
 2. The positive electrode active materialaccording to claim 1, wherein M is at least one element selected from agroup consisting of Ru, Mo, S, W, Os, and Po.
 3. The positive electrodeactive material according to claim 1, wherein M is Ru.
 4. The positiveelectrode active material according to claim 1, wherein M′ isMn_(a)M″_(1-a) (where M″ is at least one element selected from a groupconsisting of Ni, Ti, Co, Al, Fe, Mg, B, Cr, Zr, Zn, and second rowtransition metals, and 0<a<1).
 5. The positive electrode active materialaccording to claim 4, wherein M″ is at least two elements selected froma group consisting of Ni, Ti, Co, Al, Fe, Mg, B, Cr, Zr, and Zn.
 6. Thepositive electrode active material according to claim 1, wherein M′ isMn_(a)Ni_(b)Co_(c) (where 0<a<1, 0<b<1, 0<c<1, and a+b+c=1).
 7. Thepositive electrode active material according to claim 4, wherein0<a<0.5.
 8. The positive electrode active material according to claim 1,wherein 0<x≦0.7.
 9. The positive electrode active material according toclaim 1, wherein 0.4<y<1.
 10. The positive electrode active materialaccording to claim 1, wherein A and A′ are each at least one elementselected from a group consisting of halogen elements, sulfur, andnitrogen.
 11. The positive electrode active material according to claim1, wherein 0≦z≦0.2 and 0≦z′≦0.2.
 12. The positive electrode activematerial according to claim 1, wherein the compounds represented byFormula 1 above are layered composites or solid solutions.
 13. Apositive electrode formed by applying a positive electrode activematerial according to claim 1 to a current collector.
 14. A lithiumsecondary battery comprising a positive electrode according to claim 13.15. A battery module comprising a lithium secondary battery according toclaim 14 as a unit battery.
 16. A battery pack comprising a batterymodule according to claim
 15. 17. A device using a battery packaccording to claim 16 as a power source.
 18. The device according toclaim 17, wherein the device is an electric vehicle, a hybrid electricvehicle, a plug-in hybrid electric vehicle, or a power storageapparatus.
 19. The positive electrode active material according to claim6, wherein 0<a<0.5.